1. Field of the Invention
The invention relates to a device for transmitting information by means of light guides, in which a transmitter and/or a receiver are adapted to be moved or differently positioned along a light guide. Similarly, however, the transmitter and the receiver may be disposed at given fixed positions.
Devices of this kind having a linear configuration are employed for example in crane or other conveyor systems for data transmission between a movable crane and a stationary control unit. Another field of application of these data transmission systems having a circular configuration is that of transmission between parts that are rotatable relative to each other in a computer tomograph, for example, between a rotor supporting both an X-ray tube and a detector, and a stationary evaluation unit for processing and displaying image information. In stationary applications too, for example on optical circuit boards, various taps are needed along a given length. Optionally, these taps may be provided to be permanent or, for example, occupied by plug-in modules as instantly needed.
2. Description of the Prior Art
In prior art transmission systems based on light guides, light is coupled into a light guide at one end, and then passed through the light guide to the other end thereof, where it is evaluated by a suitable receiver. A multitude of different variants based on this are known, and make possible a simultaneous transmission of a plurality of wavelengths by means of filters, or even a transmission to a plurality of different sites, for example by means of Y couplers. However, systems of this kind are not suitable for coupling signals in or out laterally at any desired position along a light-guiding fiber. Various other techniques are known for this.
A device of this kind is described in the U.S. Pat. No. 5,297,225. In this, light which is coupled into the light-guiding medium from the outside through notches provided on the outside is deflected by reflection at such angles that it can be guided inside the medium. A transmission device of this kind may be used expediently when coupling-in is to be made at given fixed positions. Basically it may be used also for a transmission between movable units, because the light is coupled in or out without contact. However, if a long distance of movement is needed, as is the case with crane systems or even rotary transmission systems of large diameter, then a very high attenuation results from the many notches along the light-guiding medium. This makes extreme demands on the dynamic ratio of the receiver. A typical attenuation of this arrangement of about 10 dB per meter overtaxes conventional optical receivers at a track length of several meters. Therefore this transmission system can be put into practice for long transmission tracks only with optical components which are particularly expensive and require much outlay.
Alternatively, an example of a light guide also suitable for transmission is substantially formed by a trench having a mirror surface. A light guide of this kind is disclosed in U.S. Pat. No. 4,525,025.
The various principles of coupling light in or out involve a limited bandwidth, and are therefore hardly applicable for data rates in a Giga-baud range. If broadband light guides such as, for example, single-mode fibers are used, then the light to be coupled-in must be positioned with extreme accuracy, i.e. to within a range of micrometers. This is technically hardly feasible with systems moving rapidly with respect to each other, as in computer tomographs, where they at present move at a circumferential speed of up to 20 m/s. Even in slowly moving systems, a positioning which satisfies the usual demands of shock resistance and dynamic strength, as made on industrial and military systems, can be achieved only with extreme mechanical outlay.
If in a case of light being coupled into a light guide, attempts are made to illuminate the light guide using a beam of large cross-section, then very high transmitter power is needed for this. However, suitable high power building components, such as lasers, can usually be modulated only slowly. If in a case of a beam being coupled out, receiving diodes of large area are used, then owing to their large area here too a substantially smaller data rate is achieved.
In DE 196 25 870 A1 another optical transmission system is described in which the optical signal to be received can be coupled-in laterally by scattering. Here, in order to obtain a signal which at least can be evaluated, the light guide is designed to be an optical amplifier. It consists of a material having an electron arrangement which can be inverted by energetic excitation and which emits light by stimulated emission. In this, the stimulation is effected by light coupled in by scattering. This device requires a particularly large technical outlay. Thus, at least one pumped light source, usually a laser of high power, and also wavelength selective coupling elements for coupling-in the pumped light, and also for separating pumped light and emitted light, are needed. These additional components increase system costs of a transmission system of this kind so that it is not usable on a broad basis in industrial and medical applications.
The devices according to prior art require extremely precise focusing of the coupled-in light owing to the low efficiency of coupling-in, and thus make very high demands on the mechanical design of the entire arrangement. Apart from the mechanical problems, the known optical solutions are particularly sensitive to a contamination of the optical systems. If, for example, in the course of time the light guide becomes covered with a layer of carbon dust, for example from adjacent carbon brushes for electrical signal transmission, and also of oil and other impurities, then the attenuation of the transmission systems strongly increases.
Because the technical operating principles of rotary transmission and linear or curve-shaped transmission are the same, the following explanations will make no further distinction between them. The terms are used synonymously. Furthermore, no distinction will be made between components termed as light wave guides and light guides, because their functions are comparable as far as the invention is concerned, although light wave guides are preferably used in technical practice.